It is known that tertiary phosphines are synthesizable by various techniques, including the reduction of tertiary phosphine oxides. The tertiary phosphine oxide reduction processes that have been used in the past have included the processes of U.S. Pat. Nos. 3,261,871 (Fritzche et al. I), 3,280,195 (Fritzche et al. II), 3,847,999 (Gardner et al.), 4,008,282 (Townsend et al.), 4,113,783 (Malpass et al.), and 4,131,624 (Davis et al.); Issleib et al., CA 53:9879c; Fritzche et al. III, CA 61:8335f; Koester et al., CA 63:8398h; and G. M. Kosolapoff et al., Organic Phosphorus Compounds, Vol. 1, Wiley Interscience Publishers (New York), 1972, pp. 45-47, as well as processes wherein the oxide has been converted to the tertiary phosphine via an intermediate.
Koester et al. teach that triphenylphosphine can be obtained in a 96.2% yield by reducing triphenylphosphine oxide with a trialkylborane at 250.degree. C. for 5 hours. As indicated by the amounts of reactants employed, this reducing agent is inefficient and has to be used in a borane/oxide mol ratio of about 3/1 because only one of its three alkyl groups is utilized. Such a large amount of trialkylborane is economically unattractive in view of the high cost of trialkylboranes. Moreover, repetitions of the experiment of Koester et al. have shown that yields considerably lower than 96.2% are apt to be obtained in spite of this large amount of trialkylborane.
It would be advantageous to find a way of reducing tertiary phosphine oxides that would utilize less expensive and more efficient sources of the boron and alkyl moieties of a trialkylborane.